Transit: When Planets Cross the Sun

Transits are rare but interesting. Nobody alive has seen a transit of Venus - the planet crossing the face of the Sun - because it last happened in 1882. The next time will be in 2004.

Only Mercury and Venus orbit the Sun closer than we do and so occasionally cross the Sun's disc. Much more commonly, we can observe the moons of Jupiter and Saturn in transit, and accurate recording and imaging of these events are well within the scope of amateur astronomers.

"Transit" is in two parts. The first tells the story - always fascinating and sometimes bizarre - of the first scientific expeditions to observe transits, made in an attempt to use transit timings to accurately define the Astronomical Unit.

The second part is for practical amateur observers, and explains how to observe transits of all sorts - the rare transits of Mercury and Venus, transits of the moons of the major planets, and even transits of aircraft as they fly between the observer and the Sun!